Sulfide solid electrolyte materials exhibit high Li ion conductivity to be useful in achieving a higher output of the battery. As a raw material of sulfide solid electrolyte materials, for example, lithium compositions containing Li2S and LiI are known.
A method for synthesizing a lithium composition is disclosed in Patent Literature 1 (Japanese Patent Application Laid-Open No. 2014-179265) in which a raw material mixture containing LiHS and LiX (X represents F, Cl, Br, or I) is prepared from a single Li source and LiHS in the raw material mixture is converted into Li2S by eliminating hydrogen sulfide. In addition, a method for obtaining the raw material mixture is disclosed in Patent Literature 1 in which a reduction treatment to fabricate an aqueous solution containing a hydroxide of Me and a halide of Me by reacting a hydroxide of a metal (Me) (such as calcium hydroxide), halogen (such as iodine), and formic acid with one another, a substitution treatment to obtain lithium hydroxide and lithium halide by substituting Me with Li, and sulfurization treatment to obtain lithium hydrosulfide by sulfurizing lithium hydroxide are conducted.
Lithium compositions are desired to contain fewer impurities since they are used as, for example, a raw material of a sulfide solid electrolyte material. However, in conventional methods for producing lithium compositions, polysulfides are generated during the sulfurization treatment in some cases.
Polysulfides (such as HSn− and Sn2−) are those in which an indefinite amount of sulfur is joined in a linear or cyclic form, and it is believed that the sulfur colloid is produced as a particulate product derived from the polysulfide. Accordingly a method for producing a lithium composition which can suppress the generation of polysulfides is desired.